In many applications, a radiation source (such as a laser) or a detector (such as a focal plane array or FPA) is maintained stationary, and the output beam of the radiation source or the input beam of the detector is aimed in a desired direction using a reflective and/or refractive beam-steering apparatus. Thus, for example, an image generator may remain stationary, and the output image beam is aimed by movable mirrors and/or lenses. Similarly, an imaging focal plane array detector may remain stationary, and the area of space to be imaged is selected by movable mirrors and/or lenses.
Familiar types of beam-steering apparatus include azimuth-elevation, roll-nod, and roll-pitch gimbals. The rotation of the gimbaled structure about two axes allows the line-of-sight beam output or the line-of-sight beam input direction to be selected for an available range of elevations and azimuths. The elevation and azimuth represent two angular degrees of freedom.
A more complex problem arises where the line-of-sight must be steered in more than two degrees of freedom, and as many as five degrees of freedom. These additional degrees of freedom can include a rotational degree of freedom, that is, the rotation of the image about the line of sight. There may also be a requirement for steering one or two lateral-position degrees of freedom. These lateral-position degrees of freedom include the lateral movement of the line of sight, such as the lateral position of an entrance or exit pupil.
Control of the rotational of the line of sight about the beam axis is an important consideration in imaging systems. For example, if the system includes a focal plane array detector and the movement of the imaging optics causes the image on the focal plane array to rotate, the analysis of the image may become more difficult. Optical de-rotation devices such as K-mirrors or Pechan prisms have been developed to control the rotation of the image about the line of sight. Such de-rotation devices are typically used serially with multiple mirrors that provide two-dimensional angular beam steering and two lateral-position degrees of freedom. The resulting apparatus that provides five degrees of freedom of the beam steering is large, heavy, and complex, because it requires that a portion of the apparatus rotates as an assembly about the line of sight.
There is a need for a beam-steering apparatus that provides up to five degrees of beam-steering freedom, yet is relatively small, light, and simple in construction. The present invention fulfills this need, and further provides related advantages.